Various materials have been reported as an electrochromic (which is sometimes abbreviated as “EC” below) material in which the light absorption properties (coloration state and light transmittance) of a substance change due to an electrochemical redox reaction. Those containing metal oxides, such as WO3, have been known as inorganic EC materials but a forming method is limited to a vapor deposition method and the like, which has caused a problem in producing a large area device.
On the other hand, as organic EC materials, conductive polymers, such as polythiophene and polyaniline, organic low molecular weight compounds, such as viologen and oligothiophene, and the like have been known. Herein, examples of the organic low molecular weight EC compounds include viologen derivatives and quinone derivatives which are cathodic compounds which are colored by reduction, oligothiophene derivatives and phenazine derivatives which are anodic compounds which are colored by oxidation, and the like.
The organic low molecule weight EC compounds have been known to have higher transparency in decoloration as compared with conductive polymer EC compounds.
PTL 1 describes a phenazine derivative as the anodic EC compound which is colored in an oxidized state. PTL 2 describes a viologen derivative as the cathodic EC compound. NPL 1 describes an oligothiophene derivative as the anodic EC compound. PTL 3 describes a dithienothiophene derivative as an anode material.
As an EC element containing the organic low molecule weight EC compound, a so-called sandwich type complementary element in which a solution obtained by dissolving an anodic EC compound and a cathodic EC compound is disposed between a pair of electrodes has been known.
Since the anodic EC compound has abundant electrons in the molecules, the anodic EC compound is easily oxidized. As a result, the anodic EC compound has an electron donor property. On the other hand, since electrons are insufficient in the molecules in the cathodic EC compound, the cathodic EC compound is easily reduced. As a result, the cathodic EC compound has an electron acceptor property. Therefore, when both the EC compounds are mixed and dissolved, electron donor molecules and electron acceptor molecules interact to form a charge transfer (CT) complex, and then light absorption based on the charge transfer complex appears in a visible region. Even in the case where the anodic EC compound and the cathodic EC compound each individually have sufficient colorless transparency in decoloration, when the anodic EC compound and the cathodic EC compound are mixed, a problem that CT absorption appears in a visible region and the transparency decreases has occurred.
PTL 1 describes that the CT light absorption can be reduced in a solution containing viologen by a 5,10-dihydrophenazine derivative of a specific structure which is the anodic EC compound.